Which is the lightest and the most abundant element in the universe?

Hydrogen, with atomic number 1, is the simplest of all elements. Its common atom has just one proton and one electron, so it is also the lightest element. Its atomic mass is about 1.008 u.

Hydrogen is also the most abundant element in the universe. About three-quarters of all ordinary (baryonic) matter in the universe, by mass, is hydrogen. Most of the rest is helium. Hydrogen and helium were formed soon after the Big Bang, and heavier elements were made later inside stars.

Stars such as the Sun shine because of nuclear fusion. In the core of the Sun, hydrogen nuclei fuse to form helium, and a small amount of mass is converted into a huge amount of energy, as given by Einstein's equation E = mc2. This is why the Sun is described as being made mainly of hydrogen and helium.

On Earth, hydrogen is found mostly in combined form, especially in water (H2O) and in organic compounds such as hydrocarbons. Free hydrogen gas (H2) is colourless, odourless and highly flammable. It burns with a pale blue flame to form water, so it is called a clean fuel. Green hydrogen, made by splitting water using renewable electricity, is being promoted in India under the National Green Hydrogen Mission (2023).

Henry Cavendish identified hydrogen as a separate substance in 1766, and Antoine Lavoisier gave it its name, meaning "water-former".

Why not the other options?

• ✗Helium is the second lightest and the second most abundant element in the universe.
• ✗Oxygen is the most abundant element in the Earth's crust, not in the universe.
• ✗Carbon is the basis of organic compounds but is far less abundant than hydrogen.